You will be able to run the additional fans without causing introducing any danger to your system. The fans may not spin at full rpm, but you will want to test them to see what the results are.
Another option would be to run the fans off of the motherboard headers, but you may need to get adapters for a molex-to-3-pin connection.
Another option would be to run the fans off of the motherboard headers, but you may need to get adapters for a molex-to-3-pin connection.